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Notes and workarounds for accessing the Mac OS X pasteboard in tmux sessions. Note: The pu branch (“Proposed Updates”) may be rewound without notice.


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Quick Summary

  • Using the Mac OS X programs pbpaste and pbcopy under old versions of tmux may not work.
    Other services and unpatched builds of screen are also affected.

  • You may not even need the program from this repository!

    tmux version 2.6 (2017 September) incorporated the functionality of the wrapper program. If you are using a new enough tmux, you may be able to skip using the wrapper in your tmux configuration.

  • Certain undocumented, private API functions can fix the problem.

  • Because the functions are private, undocumented, and unstable (one acquired an extra argument in 10.6), I think using a small wrapper program might be better than patching tmux.

Thus, my wrapper-based workaround:

  1. Compile reattach-to-user-namespace from this repository.
    Make it available in your PATH (or use the absolute pathname in the next step).

     make reattach-to-user-namespace &&
     cp reattach-to-user-namespace ~/bin

    Alternatively, you can install it with MacPorts or Homebrew:

     port install tmux-pasteboard
     brew install reattach-to-user-namespace
  1. Configure tmux to use this wrapper program to start the shell for each new window.

    In .tmux.conf:

     set-option -g default-command "reattach-to-user-namespace -l zsh"

    See the file for other configuration and usage techniques (conditional use for cross-platform configurations, fine-grained usage, etc.)

  2. Restart your tmux server (or start a new one, or just reconfigure your existing one).

    To kill your existing tmux server (and everything running “inside” it!):

     tmux kill-server
  3. Enjoy being able to use pbpaste, pbcopy, etc. in new shell windows.

Purpose of These Programs

The programs in this repository were created to diagnose and enable reliable access to the Mac OS X pasteboard for programs run under tmux and unmodified versions of screen.

See the “Beyond Pasteboard Access” section of the file for other programs that benefit from the wrapper.

Mac OS X Pasteboard Access Under tmux And screen

The Problem

The most commonly reported broken behavior is that the pbcopy and pbpaste command-line programs that come with Mac OS X fail to function properly when run under tmux and (sometimes) under “unpatched” screen.

Apple has patched their builds of screen (included with Mac OS X) to fix the problem; the screen “port” in the MacPorts system has adopted Apple’s screen patches.

Their screen patch allows (for example) the user to create a screen session under a normal GUI login session and access the pasteboard (inside the screen session) anytime that user is logged into the GUI. Programs that are run in a session of “unpatched” screen will only encounter the problem when the screen session outlives its parent Mac OS X login session (e.g. a normal GUI login or an SSH login).

Third-party programs (run under tmux or unpatched screen) are also affected (e.g. non-GUI builds of Vim 7.3 can access the pasteboard when compiled with the +clipboard feature).

Why Pasteboard Breaks

Access to the Mac OS X Pasteboard Service

The pasteboard service in Mac OS X is registered in a "bootstrap namespace" (see Apple’s TN2083). The namespaces exist in a hierarchy: “higher” namespaces include access to “lower” namespaces. A process in a lower namespace can not access higher namespaces. So, all process can access the lowest, “root” bootstrap namespace, but only processes in a higher namespace can access that namespace. Processes created as a part of a Mac OS X login session are automatically included in the user’s “per-user” bootstrap namespace. The pasteboard service is only available to processes in the per-user bootstrap namespace.

Interaction with tmux

tmux uses the daemon(3) library function when starting its server process. In Mac OS X 10.5, Apple changed daemon(3) to move the resulting process from its original bootstrap namespace to the root bootstrap namespace. This means that the tmux server, and its children, will automatically and uncontrollably lose access to what would have been their original bootstrap namespace (i.e. the one that has access to the pasteboard service).

Interaction with Unpatched screen

The situation with screen is a bit different since it does not use daemon(3). Unpatched screen, and its children, only lose access to the per-user bootstrap namespace when its parent login session exits.

Solution Space

Apple (and MacPorts) have already handled screen. Apple prevents screen from losing access to the per-user bootstrap namespace by “migrating to [the] background session” (in 10.5 using _vprocmgr_move_subset_to_user) or “detach[ing] from console” (in 10.6 using _vprocmgr_detach_from_console). For the purposes of screen, both of these let the screen process access the per-user bootstrap namespace even after the processes initial Mac OS X login session has ended.

Patch tmux?

Ideally, we could port Apple’s patch to tmux. Practically, there are problems with a direct port.

The undocumented, private function used in Apple’s 10.6 patch, _vprocmgr_detach_from_console, is not effective if called before daemon(3) (since it forcibly moves the process to the root bootstrap namespace); if called after daemon(3), it just returns an error.

The undocumented, private function used in Apple’s 10.5 patch, _vprocmgr_move_subset_to_user, is also available in 10.6 (though an extra parameter has been added to it in 10.6). Again, there is no point in calling it before daemon(3), but it is effective if called after daemon(3).

The functionality of _vprocmgr_move_subset_to_user seems to be a sort of superset of that of _vprocmgr_detach_from_console in that both move to the "Background" session, but the former does some extra work that can attach to a user namespace even if the process has been previously moved out of it.

So, another approach that works is to call either the private function after invoking a custom daemon that does not forcibly move its resulting process to the root bootstrap namespace (tmux even already has one).

The fact that the signature of _vprocmgr_move_subset_to_user changed between 10.5 and 10.6 is a strong indication that Apple sees these functions as part of a private API that is liable to change or become available in any (major?) release. It seems inappropriate to ask upstream tmux to incorporate calls to functions such as these. It might be appropriate for MacPorts to apply a patch to its port though.

Use a “Reattaching” Wrapper Program

While it would be nice to have the tmux server itself reattached to the per-user bootstrap namespace, it is probably enough to selectively reattach just some of its children. A small wrapper could do the work of reattaching to the appropriate namespace and then execing some other program that will (eventually) need access to the per-user namespace.

Such a wrapper could be used to run pbcopy, pbpaste, vim, et cetera. This would require the user to remember to use the wrapper (or write scripts/shell-functions/aliases to always do it; or notice it fail then re-run it under the wrapper).

A more automated solution that probably covers most of the problem scenarios for most users would be to set tmux’s default-command option so that new windows start shells via the wrapper by default. The major area this would not cover would be commands given directly to new-session and new-window (there are some other commands that start new children, but those are the major ones).

Some New Programs For Your Consideration

The Wrapper Program

The reattach-to-user-namespace program implements the “wrapper” solution described above.

    reattach-to-user-namespace program args...

Its -l option causes it to rewrite the execed program’s argv[0] to start with a dash (-). Most shells take this as a signal that they should start as “login” shells.

    exec reattach-to-user-namespace -l "$SHELL"

In .tmux.conf:

    set-option -g default-command "reattach-to-user-namespace -l zsh"

The Diagnostic Program

The test program was created to easily examine the effects and interactions of some of the “functions of interest” (primarily daemon(3), and the private “vproc” functions).

Its arguments are interpreted as instructions to call various functions and/or display some result.


Emulate calling pbpaste under plain tmux:

    ./test daemon=sys system=pbpaste

Emulate a tmux patch that would automatically reattach to the user namespace (also equivalent to using the wrapper program under an unpatched tmux):

    ./test daemon=sys move-to-user=10.6 system=pbpaste

Emulate a tmux patch that uses compat/daemon.c and “detaches from the console”:

    ./test daemon=ours deatch system=pbpaste

Demonstrate revocation of access to the per-user bootstrap namespace when the Mac OS X login session ends:

    # while logged into the GUI

    # login session ends before pbpaste happens: failure
    cp /dev/null /tmp/f &&
    ssh localhost `pwd`/test \
      daemon=ours \
      msg=sleeping... sleep=1 msg='done\ sleeping' \
      system=pbpaste 2\> /tmp/f &&
    { cat /tmp/f; tail -f /tmp/f; }

    # pbpaste happens before login session ends: success
    cp /dev/null /tmp/f &&
    ssh localhost `pwd`/test \
      daemon=ours \
      msg=sleeping... msg='done\ sleeping' \
      system=pbpaste 2\> /tmp/f \; sleep 1 &&
    { cat /tmp/f; tail -f /tmp/f; }

Test workarounds to prevent the above end-of-login revocation:

    # while logged into the GUI

    # emulate tmux patched to move to the user namespace
    # or, equivalently, unpatched *tmux* and wrapper
    cp /dev/null /tmp/f &&
    ssh localhost `pwd`/test \
      daemon=sys \
      move-to-user=10.6 \
      msg=sleeping... sleep=1 msg='done\ sleeping' \
      system=pbpaste 2\> /tmp/f &&
    { cat /tmp/f; tail -f /tmp/f; }

    # emuate tmux patched to use compat/daemon + detach
    cp /dev/null /tmp/f &&
    ssh localhost `pwd`/test \
      daemon=ours \
      detach \
      msg=sleeping... sleep=1 msg='done\ sleeping' \
      system=pbpaste 2\> /tmp/f &&
    { cat /tmp/f; tail -f /tmp/f; }


Notes and workarounds for accessing the Mac OS X pasteboard in tmux sessions. Note: The pu branch (“Proposed Updates”) may be rewound without notice.